1. Field of the Invention
This invention relates to a method for forming a fluororesin film which has good adhesiveness and durability and wherein when a once-formed film is removed rom a substrate and a fresh one is again formed on the substrate, the re-information is feasible without damaging the substrate. The invention also relates to articles on which a fluororesin film is formed by the method.
2. Description of the Prior Art
Film formation, on metal substrate surfaces, of fluorocarbon resins or fluororesins such as polytetrafluoroethylene (PTFE), tetrafluoroethylene/hexafluoropropylene copolymers (FEP), tetrafluoroethylene/perfluoroalkyl vinyl ether copolymers and the like has been widely made on from domestic to various industrial articles in order to impart thereto non-tackiness, lubricity and corrosion resistance.
These fluororesin film cannot be formed on metal substrates in good adhesion according to ordinary coating techniques. In order to form fluororesin films on the surface of metal substrates, usual practice includes a procedure which comprises roughening the substrate surface such as by sand blasting, washing the roughened surface, applying and baking a primer so as to ensure good adhesion between the substrate surface and a fluororesin, optionally applying an intermediate coating on the primer, and finally applying and baking a finish coating of the fluororesin.
However, the fluororesin film formed by the method set out above usually consists of three or more layers including the primer layer, the intermediate layer and the finish layer. Accordingly, the total thickness of the film becomes great, which will in some cases lead to a lowering of the dimensional accuracy of the substrate.
Since the fluororesin has good non-adhesiveness and good lubricant property, the adhesion to the substrate is not always satisfactory when the film is formed by the above-stated method. This presents not only the problem that the film is disadvantageously apt to be separated during use, but also the problem that the hardness is not so high and the film is liable to suffer abrasion.
In case where the fluororesin film is degraded owing to the separation or abrasion, the degraded film is removed from the substrate. This may require re-formation of a fresh film. To this end, there is known a method wherein a once formed film is, for example, physically removed such as by sand blasting and a fresh film is again formed on the removed surface. The removal of the film by the physical method inevitably damages the substrate surfaces, with the possibility that the substrate is not used in industrial articles requiring a fine dimensional accuracy such as, for example, molds.